NAME

cdb - Constant DataBase file format

DESCRIPTION

A cdb database is a single file used to map ‘keys’ to ‘values’, having
records of (key,value) pairs. File consists of 3 parts: toc (table of
contents), data and index (hash tables).
Toc has fixed length of 2048 bytes, containing 256 pointers to hash
tables inside index sections. Every pointer consists of position of a
hash table in bytes from the beginning of a file, and a size of a hash
table in entries, both are 4-bytes (32 bits) unsigned integers in
little-endian form. Hash table length may have zero length, meaning
that corresponding hash table is empty.
Right after toc section, data section follows without any alingment.
It consists of series of records, each is a key length, value (data)
length, key and value. Again, key and value length are 4-byte unsigned
integers. Each next record follows previous without any special
alignment.
After data section, index (hash tables) section follows. It should be
looked to in conjunction with toc section, where each of max 256 hash
tables are defined. Index section consists of series of hash tables,
with starting position and length defined in toc section. Every hash
table is a sequence of records each holds two numbers: key’s hash value
and record position inside data section (bytes from the beginning of a
file to first byte of key length starting data record). If record
position is zero, then this is an empty hash table slot, pointed to
nowhere.
CDB hash function is
hv = ((hv << 5) + hv) ^ c
for every single c byte of a key, starting with hv = 5381.
Toc section indexed by (hv % 256), i.e. hash value modulo 256 (number
of entries in toc section).
In order to find a record, one should: first, compute the hash value
(hv) of a key. Second, look to hash table number hv modulo 256. If it
is empty, then there is no such key exists. If it is not empty, then
third, loop by slots inside that hash table, starting from slot with
number hv divided by 256 modulo length of that table, or ((hv / 256) %
htlen), searching for this hv in hash table. Stop search on empty slot
(if record position is zero) or when all slots was probed (note cyclic
search, jumping from end to beginning of a table). When hash value in
question is found in hash table, look to key of corresponding record,
comparing it with key in question. If them of the same length and
equals to each other, then record is found, overwise, repeat with next
hash table slot. Note that there may be several records with the same
key.